1. Field of the Invention
The present invention relates to a digital camera of a single-lens-reflex type, and more particularly to a digital camera that automatically adjusts the amount of light in flash shooting.
2. Description of the Prior Art
Generally, a digital camera is equipped with a display device such as a liquid crystal display device, and this display device is used both to display a playback of an image shot and stored previously and to display an image just being shot. This enables the user to determine picture composition and confirm focus condition while viewing a “live view”, i.e. the image just being shot, that is displayed on the display device. Thus, the display device functions as a video viewfinder.
Some digital cameras are additionally equipped with an optical viewfinder. Digital cameras equipped with an optical viewfinder divide into a separate type, in which light is introduced to the viewfinder without being passed through a taking lens, and a single-lens-reflex (SLR) type, in which the light having passed through a taking lens is reflected so as to be introduced to the viewfinder. Whereas the separate type suffers parallax between the viewfinder and the taking lens, the SLR type has the advantage of intrinsically being free of parallax.
The SLR type requires a prism to be disposed in the optical path from the taking lens to an image-sensing device so as to reflect a part and transmit the other part of the light having passed through the taking lens, and requires also a pentaprism for directing the reflected light to the eye of the user in such a way as to permit the user to observe an erect image. In this arrangement, the optical viewfinder and the video viewfinder can be used at the same time. However, only part of the light having passed through the taking lens reaches the image-sensing device, and therefore light is used inefficiently in shooting.
As is well known, in an SLR-type camera that shoots by exposure of silver-halide film, it has been customary to dispose a total-reflection mirror obliquely in the optical path from the taking lens to the film in such a way that the mirror can be rotated out of the optical path. Until immediately before shooting, i.e. exposure of film, the mirror is kept in the optical path so that the incoming light is reflected toward the pentaprism. Only at the moment of shooting is the mirror retracted out of the optical path so that the incoming light is directed to the film, and, immediately after shooting, the mirror is brought back in the optical path. This type of mirror is called a quick-return mirror.
Digital cameras equipped with such a quick-return mirror are already in practical use, producing bright images despite using an optical viewfinder. However, in this arrangement, while the total-reflection mirror is placed in the optical path to permit the use of the optical viewfinder, no light reaches the image-sensing device, and thus the display device cannot be used as a viewfinder.
To overcome this inconvenience, digital cameras equipped with a half mirror as a quick-return mirror have been proposed. Whether the half mirror is placed in or retracted out of the optical path, the incoming light reaches the image-sensing device, and therefore both shooting and display are possible at any time. While the half mirror is placed in the optical path, the optical viewfinder can be used as well. On the other hand, when an image to be recorded is shot, the half mirror is retracted out of the optical path so that all the light from the taking lens is used for shooting.
Generally, irrespective of the type of camera, when shooting is attempted in so low light that bright images are unlikely to be obtained, flash is used so that the subject is illuminated to proper brightness with illumination light emitted from a flash device. To permit proper adjustment of the amount of illumination light, some SLR-type cameras that shoot by exposure of silver-halide film adopt control called automatic light adjustment. Specifically, the amount of illumination light that reaches the film during shooting is measured, and emission of the illumination light is stopped when the measured amount has reached a predetermined value. This control works excellently to permit the subject to be illuminated to proper brightness.
Such automatic light adjustment requires the use of a photometric device, which is usually disposed by the side of the optical path from the talking lens to the film in such a way as to face the film obliquely, so that the photometric device measures the amount of light reflected from the film. Since the film reflects light dispersively, disposing the photometric device in this way suffices to enable it to measure the amount of reflected light.
On the other hand, in a digital camera, the image-sensing device itself outputs a signal representing the amount of light received, and therefore exposure of the image-sensing device can be controlled on the basis of that signal. In fact, in ambient-light shooting involving no emission of illumination light, on the basis of the output of the image-sensing device, the aperture value and the exposure time (electronic shutter speed) to be used in succeeding shooting are determined. However, since it is only after the completion of shooting of the image of a frame that the image-sensing device outputs the signal corresponding to that frame, it is impossible to use the image-sensing device as a photometric device for the purpose of automatic light adjustment.
For this reason, in a digital camera, first, an image is shot with preliminarily emitted illumination light and, on the basis of the result of this shooting, the amount of illumination light to be emitted is determined. It is only then that an image to be recorded is shot with emission of the thus determined amount of illumination light. However, this control has the disadvantage of requiring too much time after the user has operated the release button to command recording of an image until the image to be recorded starts to be shot.
Some might expect that, even in a digital camera, if it is additionally equipped with a photometric device separate from an image-sensing device, it would be possible to realize automatic light adjustment that permits accurate adjustment of the amount of light with quick response. In reality, however, since the image-sensing device used in a digital camera hardly reflects light dispersively, it is meaningless to dispose the photometric device to face the image-sensing device obliquely. Also meaningless is to dispose the photometric device to face the image-sensing device squarely, because then the photometric device obstructs part of the light traveling from the taking lens to the image-sensing device.
On the other hand, in an SLR-type digital camera, as described above, the optical path is split by a semi-transmissive optical element such as a prism or half-mirror so that the light reflected from that optical element is directed to the optical viewfinder. However, no arrangement has conventionally been known that uses this reflected light for any other purpose than to offer a viewfinder image.